1. Field of Invention
The present invention relates to a technical field of a driving circuit and a driving method for driving an electro-optical device such as a liquid crystal device, the electro-optical device, and electronic equipment employing the electro-optical device and, more particularly, to a driving circuit and a driving method of an electro-optical device that receives a digital image signal and has a DA (Digital to Analog) converting function and a xcex3 correcting function for an electro-optical device, the electro-optical device, and electronic equipment using the electro-optical device.
2. Description of Related Art
Hitherto, as a driving circuit for driving a liquid crystal device, which is an example of one type of electro-optical device, there is available, for example, a so-called digital driving circuit configured to receive digital image data indicating an arbitrary step of gray scale among a plurality of steps of gray scale, generate analog image data having a driving voltage corresponding to the step of gray scale, and supply the generated analog image data to a signal line of the liquid crystal device. Such a driving circuit is usually provided with a digital-to-analog converter (hereinafter referred to as xe2x80x9cDA converterxe2x80x9d or xe2x80x9cDACxe2x80x9d as necessary) for converting digital image data to analog image data; it is configured to latch the digital image data, which has been input via a digital interface, by a latching circuit, then subject it to analog conversion through a switched capacitor type DA converter (hereinafter referred to as xe2x80x9cSC-DACxe2x80x9d (Switched Capacitor-DAC: switch control capacity type DAC) as necessary), a DAC composed of a resistance ladder circuit or the like.
In a liquid crystal device or the like, the changes in optical characteristics (transmittance, optical density, luminance or the like) with respect to the changes in the driving voltage (or a voltage applied to the liquid crystal) are generally nonlinear according to the saturation characteristic or threshold value characteristic that the liquid crystal or the like has and they exhibit a so-called xe2x80x9cxcex3 characteristic.xe2x80x9d Hence, this type of driving circuit is normally provided with xcex3 correcting means for making a correction on digital image data in a stage preceding the latching circuit.
The xcex3 correcting means, for example, carries out xcex3 correction on 6-bit digital image data DA by referring to a table stored in RAM or ROM so as to convert it into 8-bit digital image data DB (Dxcex31, Dxcex32, . . . , Dxcex38). The processing by the xcex3 correcting means is implemented, considering the input/output characteristics of the DAC and the characteristic of the transmittance of liquid crystal pixels with respect to the voltage applied to a signal line (characteristics of transmittance vs. the voltage applied to liquid crystal). The transmittance characteristic of the liquid crystal pixels refers to the characteristic of changes in the transmittance of light obtained by transmitting through a liquid crystal layer with respect to the voltage applied to the liquid crystal layer held between a pair of substrates (transmitting through polarizer if they are disposed outside the substrates as necessary).
On the other hand, the aforesaid SC-DAC is constituted by a plurality of capacitive elements disposed in parallel. The respective capacitive elements have binary ratios of, for example, 20C, 2C, 22C, 24C and so on. Using these capacitive elements, a pair of reference voltages are subjected to voltage division or the like (charge share) thereby to output analog image data having a driving voltage that changes according to the changes in the gray scale of image data DB. The DAC such as the SC-DAC configured as described above is connected to a signal line of a liquid crystal device or the like; a buffer circuit or the like is provided between the output terminal of the DAC and the signal line so as to protect the output voltage from the influences of the parasitic capacitance of the signal line.
As set forth above, the driving circuit causes a voltage corresponding to the digital image data DB to be applied to the respective signal lines of a liquid crystal device or the like.
Graph (A) on the left in FIG. 21 shows the relationship between the decimal values of image data DA and output voltage Vc of the DAC; graph (B) on the right in FIG. 21 shows the relationship between transmittance SLP of liquid crystal pixels and voltage VLP applied to the signal line (the axis of the transmittance is based on the logarithm). At the center in FIG. 21, the binary values of 8-bit digital image data DB are given between the two graphs (A) and (B).
In graph (B) on the right in FIG. 21, 26 pieces of 8-bit data capable of distinguishably representing the transmittance characteristic of the liquid crystal pixels are selected among 28 pieces of 8-bit data obtained from the 8-bit input data to make the xcex3 correction and the selected pieces of data are tabulated. And when 6-bit image data DA is input, the xcex3 correcting means converts it into 8-bit data DB according to the table and outputs it to the DAC. More specifically, image data DA is represented in 64-step gray scale; therefore, the foregoing conversion is carried out so that the data DA for 64 steps of gray scale may be specified among the 256 steps of gray scale that can be represented by image data DB in order to provide even changing ratio of the transmittance in the liquid crystal when image data DA expressed in the 64-step gray scale is changed.
Thus, FIG. 21 illustrates the correspondence relationship between the 6-bit image data DA and the 8-bit image data DB and output voltage Vc (equivalent to VLP) of the DAC.
The foregoing conventional driving circuit, however, requires xcex3 correcting means and RAM or ROM or the like for storing the conversion table for the xcex3 correction which are provided in the stage preceding the latching circuit in order to make xcex3 correction. These components, therefore, provide obstacles in an attempt to reduce the size of the driving circuit. It would be possible to make up the DAC by using many amplifiers so as to provide it with the xcex3 correcting function without using the aforesaid SC-DAC. This, however, would pose such a problem as a more complicated circuit. In addition, forming operational amplifiers on a glass substrate tends to cause more variations in operating characteristics to occur.
Accordingly, it is an object of the present invention to provide a driving circuit of an electro-optical device that is compatible with digital image signals and has a relatively simple and small-scale circuit configuration to provide a DA converting function and a xcex3 correcting function (or an auxiliary function for making a xcex3 correction), the electro-optical device, and electronic equipment employing the electro-optical device.
To this end, according to one aspect of the present invention, there is provided a driving circuit of an electro-optical device that supplies an analog image signal, which has a driving voltage corresponding to an arbitrary step of gray scale among 2N (where N is a natural number) steps of gray scale, to a signal line of an electro-optical device in which the changes in the optical characteristics with respect to the changes in the driving voltage are nonlinear; the driving circuit of the electro-optical device being provided with: an input interface to which an N-bit digital image signal indicative of the arbitrary step of gray scale is applied; and a digital-to-analog converter that generates a voltage within a range of a pair of first reference voltages according to the bit value of the foregoing digital image signal to produce the driving voltage within a first driving voltage range corresponding to the step of gray scale of the digital image signal so that the changes in the driving voltage with respect to the changes in the step of gray scale of the digital image signal are nonlinear if the applied digital image signal indicates a step of gray scale from a first to mxe2x88x921th (where xe2x80x9cmxe2x80x9d is a natural number and 1 less than mxe2x89xa62N), and generates a voltage within a range of a pair of second reference voltages according to the bit value of the foregoing digital image signal to produce the driving voltage that corresponds to the step of gray scale of the digital image signal and also lies within a second driving voltage range adjacent to the first driving voltage range so that the changes in the driving voltage with respect to the changes in the gray scale of the digital image signal are nonlinear if the digital image signal indicates a step of gray scale from an mxe2x88x92th to 2Nxe2x88x92th gray scale, and supplies the analog image signal having the generated driving voltage to the signal line.
According to another aspect of the present invention, there is provided a driving method of an electro-optical device having a digital-to-analog converter that supplies an analog image signal having a driving voltage corresponding to an arbitrary step of gray scale among 2N (where N is a natural number) steps of gray scale to a signal line of the electro-optical device in which the optical characteristics thereof change nonlinearly with respect to the changes in the driving voltage, the driving method including the steps of:
inputting an N-bit digital image signal indicative of the arbitrary step of gray scale to the digital-to-analog converter;
generating, by the digital-to-analog converter, a voltage within the range of a pair of first reference voltages according to the bit value of the foregoing digital image signal to produce the driving voltage within a first driving voltage range corresponding to the step of gray scale of the digital image signal so that the changes in the driving voltage with respect to the changes in the step of gray scale of the digital image signal are nonlinear if the input digital image signal indicates a step of gray scale from a first to mxe2x88x921th (where xe2x80x9cmxe2x80x9d is a natural number and 1 less than mxe2x89xa62N;
generating, by the digital-to-analog converter, a voltage within the range of a pair of second reference voltages according to the bit value of the foregoing digital image signal to produce the driving voltage that corresponds to the step of gray scale of the digital image signal and also lies within a second driving voltage range adjacent to the first driving voltage range so that the changes in the driving voltage with respect to the changes in the gray scale of the digital image signal are nonlinear if the digital image signal indicates a step of gray scale from the mxe2x88x92th to 2Nxe2x88x92th; and
supplying the analog image signal having the generated driving voltage to the signal line.
According to the driving circuit and driving method of an electro-optical device, the N-bit digital image signal indicating an arbitrary step of gray scale is supplied first via an input interface. Then, if the supplied digital image signal indicates a step of gray scale from the first to the mxe2x88x921th, a voltage within the range of the pair of first reference voltages is selectively generated according to the bit value of the digital image signal by the digital-to-analog converter so as to produce the driving voltage that lies within the first driving voltage range. On the other hand, if the digital image signal indicates a step of gray scale from the mxe2x88x92th to the 2Nxe2x88x92th, then a voltage within the range of the pair of the second reference voltages is selectively generated according to the bit value of the digital image signal by the digital-to-analog converter so as to produce the driving voltage that lies within the second driving voltage range. And the analog image signal having the driving voltage thus generated is supplied to the signal line to drive the electro-optical device. At this time, the changes in the optical characteristics with respect to the changes in the driving voltage in the electro-optical device are nonlinear, and the changes in the driving voltage with respect to the changes in the gray scale of the digital image signal in the digital-to-analog converter are also nonlinear.
In general, the changes in the driving voltage (output) in response to the step of gray scale (input) in the digital-to-analog converter that divides the reference voltages become almost linear if the step of gray scale is low, whereas they tend to be saturated and exhibit, for example, asymptote-like nonlinearity as the step of gray scale becomes higher because of the parasitic capacitance of the signal line on the output side. On the other hand, there are cases where the changes in the optical characteristics (output) with respect to the driving voltage (input) in the electro-optical device show an S-shaped nonlinearity having its inflection point located at around the center thereof due to the saturation characteristic that most electro-optical devices have, a threshold value characteristic or the like. For instance, in the case of a liquid crystal device, the changes in the transmittance (an example of the optical characteristic) with respect to applied voltage in liquid crystal pixels exhibit the saturation characteristic in the areas in the vicinity of a maximum applied voltage and a minimum applied voltage, respectively; therefore, the changes show the S-shaped nonlinearity having its inflection point located at around the central voltage.
Accordingly, if a single reference voltage is divided in the digital-to-analog converter, it would be difficult to correct the nonlinearity of the optical characteristics (e.g. the S-shaped nonlinearity having its inflection point located at around the center thereof) in the electro-optical device by making use of the nonlinearity of the driving voltage (e.g. asymptote nonlinearity) because of the non-similarity between the two. According to the present invention, however, the nonlinearity of the driving voltage in the first driving voltage range obtained by generating the voltage within the range of the first reference voltage can be combined with the nonlinearity of the driving voltage in the second driving voltage range obtained by generating the voltage within the range of the second reference voltage so as to make the nonlinearity of the driving voltage over the entire first and second driving voltage ranges similar to a certain extent to the nonlinearity of the optical characteristics (in other words, it is possible to provide both nonlinearities with a change trend that is similar to a certain extent). In particular, by setting the voltage so that the polarities of the pair of the first reference voltages and the polarities of the pair of the second reference voltages are opposite in relation to the digital-to-analog converter, the driving voltage with respect to the gray scale can be inflected at the boundary of the first and second driving voltage ranges.
Thus, it is possible to drive the electro-optical device by using a digital image signal as an input, and to correct the nonlinearity of the optical characteristics of the electro-optical device by making use of the nonlinearity of the driving voltage of the digital-to-analog converter according to the degree of the similarity between these nonlinearities. This means that the xcex3 correction for the electro-optical device can be made by using the digital-to-analog converter.
According to the present invention as set forth above, it is not required to separately provide the xcex3 correcting means in a stage preceding the digital-to-analog converter, which was required in the prior art. As an alternative, however, such a xcex3 correcting means may be separately provided to make a xcex3 correction in a first stage, and a xcex3 correction in a second stage may be made by the foregoing digital-to-analog converter in accordance with the present invention. In this case, a rough xcex3 correction may be made in one of these two stages, then a fine xcex3 correction may be made in the other stage.
In a mode of the driving circuit in accordance with the present invention described above, the voltage polarities of the pair of the first reference voltages and the voltage polarities of the pair of the second reference voltages supplied to the digital-to-analog converter are set to be opposite from each other so that the changes in the driving voltage corresponding to the changes in the gray scale have the inflection points between the first and second driving voltage ranges.
According to this embodiment, the optical characteristics in the electro-optical device exhibit the S-shaped nonlinearity having the inflection point between the first and second driving voltage ranges. Meanwhile, the first and second reference voltages, in which the voltage polarities of the reference voltages are opposite to each other, are supplied to the digital-to-analog converter; hence, the driving voltage in the digital-to-analog converter also exhibits the S-shaped nonlinearity having the inflection point located between the first and second driving voltage ranges. Further, there is the change trend corresponding to the change in the S-shaped nonlinearity of the optical characteristics, thus making it possible to achieve a high level of correction of the nonlinearity of the optical characteristics in the electro-optical device by utilizing the nonlinearity of the driving voltage over the entire first and second driving voltage ranges.
In another embodiment of the driving circuit in accordance with the present invention described above, the value of xe2x80x9cmxe2x80x9d is equal to 2Nxe2x88x921 and lower Nxe2x88x921 bits of the digital image signal are selectively input to the digital-to-analog converter as they are or after being inverted according to the value of the most significant bit of the digital image signal. The digital-to-analog converter generates a voltage in the range of the first reference voltage if the lower Nxe2x88x921 bits are input thereto as they are, and it generates a voltage in the range of the second reference voltage if the lower Nxe2x88x921 bits are inverted before being input thereto.
According to the embodiment, the value of xe2x80x9cmxe2x80x9d is equal to 2Nxe2x88x921. In other words, the first half or the latter half of the 2N steps of gray scale corresponds to the driving voltage in the first driving voltage range and the other half corresponds to the driving voltage in the second driving voltage range. In this case, lower Nxe2x88x921 bits of the digital image signal are selectively input to the digital-to-analog converter as they are or after being inverted, depending upon the binary value (i.e. depending upon whether the value is xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d) of the most significant bit of the digital image signal. The digital-to-analog converter generates a voltage in the range of the first reference voltage to generate the driving voltage in the first driving voltage range if the lower Nxe2x88x921 bits are input thereto as they are. On the other hand, the digital-to-analog converter generates a voltage in the range of the second reference voltage to generate the driving voltage in the second driving voltage range if the lower Nxe2x88x921 bits are inverted before being input thereto. Hence, only one Nxe2x88x921 bit digital-to-analog converter is required as the digital-to-analog converter for converting N-bit digital image signals, making it extremely advantageous from the viewpoint of the composition of the device.
In this embodiment, a selective inverting circuit for selectively inverting the lower Nxe2x88x921 bits depending upon the value of the most significant bit may be further provided between the interface and the digital-to-analog converter.
In such a configuration, when a digital image signal is input via the interface, the selective inverting circuit selectively inverts the lower Nxe2x88x921 bits according to the value of the most significant bit. And the selectively inverted lower Nxe2x88x921 bits are input to the digital-to-analog converter which generates a voltage in the range of the first or second reference voltage so as to generate a driving voltage in the first or second driving voltage range.
Still another embodiment of the driving circuit in accordance with the present invention is further provided with a selective voltage supply circuit for selectively supplying either the first or second reference voltage to the digital-to-analog converter according to the value of the most significant bit of the digital image signal.
According to this embodiment, depending upon the value of the most significant bit of the digital image signal, the selective voltage supply circuit selectively supplies the first or second reference voltage to the digital-to-analog converter. Then, the digital-to-analog converter generates a voltage in the range of the first or second reference voltage selectively supplied so as to generate a driving voltage in the first or second driving voltage range. Thus, the portion of the digital-to-analog converter for selectively generating a voltage in the range of the first reference voltage can be commonly used as the portion of the digital-to-analog converter for selectively generating a voltage in the range of the second reference voltage, making it advantageous from the viewpoint of the composition of the device.
Yet another embodiment of the driving circuit in accordance with the present invention is further provided with, as the digital-to-analog converter, a switched capacitor type digital-to-analog converter adapted to generate the voltages in the ranges of the first and second reference voltages, respectively, by means of charging a plurality of capacitors.
According to this embodiment, the voltages in the ranges of the first and second reference voltages are generated by the plurality of capacitors of the switched capacitor type digital-to-analog converter. This makes it possible to generate driving voltages by relatively reliable, accurate voltage selection by using a relatively simple composition.
In this embodiment, the first reference voltage may be composed of a pair of voltages that enable a voltage in the first driving voltage range to be selectively generated, and the second reference voltage may be composed of a pair of voltages that enable a voltage in the second driving voltage range to be selectively generated.
Such a composition allows a voltage in the range of a pair of the first reference voltages to be generated by the plurality of capacitors of the switched capacitor type digital-to-analog converter, thereby providing a discrete driving voltage that lies in the first driving voltage range. On the other hand, a voltage in the range of a pair of the second reference voltages is generated to provide a discrete driving voltage that lies in the second driving voltage range. Hence, desired first and second driving voltage ranges can be obtained according to the setting of the pair of the first reference voltages and the setting of the pair of the second reference voltages, and the gap between these ranges can be also reduced.
In this case, the value of the foregoing xe2x80x9cmxe2x80x9d is equal to 2Nxe2x88x921, and the composition may be such that the lower Nxe2x88x921 bits of the digital image signal are selectively input to the switched capacitor type digital-to-analog converter as they are or inverted before being input thereto according to the value of the most significant bit of the digital image signal, and the switched capacitor type digital-to-analog converter generates a voltage in the range of the first reference voltage if the lower Nxe2x88x921 bits are input thereto as they are, and it generates a voltage in the range of the second reference voltage if the lower Nxe2x88x921 bits are inverted before being input thereto.
According to the configuration set forth above, the value of xe2x80x9cmxe2x80x9d is equal to 2Nxe2x88x921, and the first half or the latter half of the 2N steps of gray scale corresponds to the driving voltage in the first driving voltage range and the other half corresponds to the driving voltage in the second driving voltage range. In this case, lower Nxe2x88x921 bits of the digital image signal are selectively input to the switched capacitor type digital-to-analog converter as they are or after being inverted depending upon the value of the most significant bit of the digital image signal. And the switched capacitor type digital-to-analog converter generates a voltage in the range of the first reference voltage to generate a driving voltage in the first driving voltage range if the lower Nxe2x88x921 bits are input thereto as they are. On the other hand, the switched capacitor type digital-to-analog converter generates a voltage in the range of the second reference voltage to generate a driving voltage in the second driving voltage range if the lower Nxe2x88x921 bits are inverted before being input thereto. Hence, only one Nxe2x88x921 bit switched capacitor type digital-to-analog converter is required as the SC-DAC to convert an N-bit digital image signal, making it extremely advantageous from the viewpoint of the composition of the device.
In this case, the switched capacitor type digital-to-analog converter may be further provided with: a first through Nxe2x88x921th capacitive elements respectively having a pair of opposed electrodes, wherein one of the paired first reference voltages or one of the paired second reference voltages is selectively applied to one of the paired opposed electrodes according to the binary value of the most significant bit; a capacitive element resetting circuit for short-circuiting the pair of opposed electrodes in each of the first through Nxe2x88x921th capacitive elements so as to discharge electric charges; a signal line potential resetting circuit for selectively resetting the voltage of the signal line to the other of the paired first reference voltages or the other of the paired second reference voltages according to the binary value of the most significant bit; and a selective switching circuit including a first through Nxe2x88x921th switches that selectively connect the first through Nxe2x88x921th capacitive elements to the signal lines, respectively, according to the values of the lower Nxe2x88x921 bits after the discharge by the capacitive element resetting circuit and the resetting by the signal line potential resetting circuit.
According to the configuration set forth above, in each of the first through Nxe2x88x921th capacitive elements, one of the paired first reference voltages or one of the paired second reference voltages is selectively applied to one of the paired opposed electrodes according to the binary value of the most significant bit. First, the pair of the opposed electrodes are short-circuited and the electric charges are discharged in each of the first through Nxe2x88x921th capacitive elements by the capacitive element resetting circuit. On the other hand, the voltage of the signal line is selectively reset to the other of the paired first reference voltages or the other of the paired second reference voltages according to the binary value of the most significant bit by the signal line potential resetting circuit. After that, the first through Nxe2x88x921th capacitive elements are selectively connected to the signal lines by the first through Nxe2x88x921th switches of the selective switch circuit in accordance with the values of the lower Nxe2x88x921 bits. As a result, the voltages (positive or negative voltages) charged in the respective capacitive elements are applied as the driving voltages to the signal lines according to the steps of gray scale indicated by a digital image signal. Thus, it is possible to generate a driving voltage, which has been selected within the ranges of the reference voltages relatively reliably and accurately, by using a relatively simple composition.
Especially in this case, each of the capacitive elements constituting the switched capacitor type digital-to-analog converter are directly connected to the signal lines and the minimum electric charges required for charging the parasitic capacitance of the signal lines can be directly supplied from each of the capacitive elements. This is extremely advantageous in reducing the power consumed by the digital-to-analog converter and the driving circuit. In particular, the power consumption can be markedly reduced in comparison with the conventional case where a buffer circuit or the like is installed between the output terminal of the switched capacitor type digital-to-analog converter and the signal line to correct the nonlinearity of the driving voltage attributable to the parasitic capacitance of the signal line.
In this case, the capacitances of the first through Nxe2x88x921th capacitive elements may be set to Cxc3x972ixe2x88x921 (C: Predetermined unit capacitance; i=1, 2, . . . , Nxe2x88x921).
This configuration makes it possible to change a driving voltage, which is obtained by selective voltage generation, at predetermined intervals so as to enable the optical characteristics in the electro-optical device to be changed at the predetermined intervals. Hence, stable multi-step gray scale can be indicated over the entire gray scale range.
In another embodiment of the driving circuit in accordance with the present invention set forth above, the values of the first and second reference voltages are set so that the difference between the driving voltage corresponding to the mxe2x88x921th step of gray scale and the driving voltage corresponding to the mxe2x88x92th step of gray scale is smaller than a predetermined value.
According to this embodiment, the difference between the driving voltage corresponding to the mxe2x88x921th step of gray scale, i.e. a driving voltage that lies within the first driving voltage range and that is closest to the second driving voltage range at the same time, and the driving voltage corresponding to the mxe2x88x92th step of gray scale, i.e. a driving voltage that lies within the second driving voltage range and that is closest to the first driving voltage range at the same time, is smaller than the predetermined value. Therefore, by setting the predetermined value to a value that has been experimentally established in advance, e.g. to a value corresponding to a difference in gray scale that cannot be recognized by human, it becomes possible to prevent a practically discontinuous change in the gray scale at the gap between the first and second driving voltage ranges (i.e. the boundary of the two ranges).
In this embodiment, the values of the first and second reference voltages may be set so that the ratio of the optical characteristics in the case where the electro-optical device is driven by the driving voltage corresponding to the mxe2x88x921th step of gray scale and the case where the electro-optical device is driven by the driving voltage corresponding to the mxe2x88x92th step of gray scale is equal to one step of gray scale obtained by dividing the variation range of the optical characteristics by (2Nxe2x88x921).
According to such a composition, the driving voltage obtained by selective voltage generation can be changed at predetermined intervals even before and after the boundary of the first and second driving voltage ranges, so that the optical characteristics in the electro-optical device can be changed at predetermined intervals. This means that highly stable multi-step gray scale display can be achieved over the entire gray scale range including the gray scale range corresponding to the boundary.
In a further embodiment of the driving circuit in accordance with the present invention described above, the digital-to-analog converter is provided with a resistance ladder that divides the first and second reference voltages, respectively, by a plurality of resistors connected in series.
According to this embodiment, the plurality of resistors of the resistance ladder generate the voltages in the ranges of the first and second reference voltages by dividing the voltages. Thus, the driving voltages can be generated relatively reliably and accurately by dividing voltages by using a relatively simple composition.
This embodiment may be further provided with a selective voltage supply circuit for selectively supplying either the first or the second reference voltage to the digital-to-analog converter according to the value of the most significant bit of the digital image signal. The digital-to-analog converter may be further provided with a decoder that decodes the lower Nxe2x88x921 bits of the digital image signal and outputs decoded signals through 2Nxe2x88x921 output terminals, and 2Nxe2x88x921 switches, one terminal of each of which is connected to each of a plurality of taps drawn out among the plurality of resistors and the other terminal thereof is connected to each of the signal lines and the 2Nxe2x88x921 switches being respectively operated according to the decoded signals output through the 2Nxe2x88x921 output terminals.
In this case, the selective voltage supply circuit selectively supplies either the first or the second reference voltage to the digital-to-analog converter according to the binary value of the most significant bit of the digital image signal. Then, in the digital-to-analog converter, the decoder decodes the lower Nxe2x88x921 bits of the digital image signal and outputs binary decoded signals respectively through the 2Nxe2x88x921 output terminals. Then, when the 2Nxe2x88x921 switches respectively connected between the plurality of taps respectively drawn out among the plurality of resistors and the signal lines are operated according to the decoded signals output through the 2Nxe2x88x921 output terminals, the first and second reference voltages are divided according to the gray scale indicated by the digital image signal. As a result, the voltages obtained by the voltage division by the respective resistors are applied as the driving voltages to the signal lines according to the gray scale indicated by the digital image signal. Thus, it becomes possible to generate a driving voltage by relatively reliable and accurate voltage division by using a relatively simple configuration.
Dividing the voltage by using the resistance ladder is especially advantageous because it eliminates the possibility of the reverse change of the driving voltage with respect to the change in the gray scale via the gap (boundary) of the first and second driving voltage ranges.
In another embodiment of the driving circuit in accordance with the present invention set forth above, the signal lines are provided with predetermined capacitors in addition to the parasitic capacitance of the signal lines.
According to this embodiment, the changes in the driving voltage (output) with respect to the changes in the gray scale (input) in the digital-to-analog converter generating voltages in the ranges of the reference voltages as previously described exhibit, for example, asymptote-shaped nonlinearity due to the parasitic capacitance of the signal lines located on the output side; therefore, adding the predetermined capacitance as mentioned above makes it possible to bring the nonlinearity of the driving voltage to a desired one or somewhat close to a desired one. The specific value of the predetermined capacitance for obtaining such desired nonlinearity may be set by carrying out experiments, simulations, or the like. Thus, the nonlinearity of the driving voltages in the first and second driving voltage ranges can be matched to each other by the nonlinearity of the optical characteristics by adjusting the additional capacitance of the signal lines in addition to the selective voltage generation carried out based on the two different reference voltages (namely, the first and second reference voltages). As a result, the nonlinearity of the optical characteristics can be corrected by making use of the nonlinearity of the driving voltage that is more similar thereto.
In a further embodiment of the driving circuit in accordance with the present invention described above, the electro-optical device is a liquid crystal device composed of liquid crystal held between a pair of substrates, and the driving circuit is formed on one of the paired substrates.
According to this embodiment, a digital image signal can be directly input, and the gray scale display on the liquid crystal device can be accomplished at relatively low power consumption by using a relatively simple configuration. Furthermore, the xcex3 correction of the liquid crystal device can be also made.
In this embodiment, each of the first and second reference voltages may be supplied to the digital-to-analog converter with the voltage polarity with respect to a predetermined reference potential being inverted for each horizontal scanning period.
According to the configuration described above, each of the voltage polarity of the first reference voltage and that of the second reference voltage is switched for each horizontal scanning period when supplying the reference voltages to allow the liquid crystal device to be driven by a scanning line reversing drive (so-called xe2x80x9c1H reversing drivexe2x80x9d) system, wherein the driving voltage is inverted for each scanning line, or a pixel reversing drive (so-called xe2x80x9cdot inverting drivexe2x80x9d) system. This prevents the flickers on a display screen and also prevents other problems such as a deterioration in liquid crystal due to the application of DC voltage. The predetermined potential providing the reference for the polarity inversion in this case is approximately equal to the opposed potential applied to one electrode of a liquid crystal pixel, to which the driving voltage supplied from the driving circuit is applied, and the other electrode opposed to the foregoing electrode via a liquid crystal layer. However, in the case of a configuration where the voltages are applied to liquid crystal pixels via switching elements such as transistors or nonlinear elements, the foregoing predetermined potential is biased with respect to the opposed potential, considering a drop in the applied voltage attributable to the parasitic capacitance of the switching elements, or the like.
To solve the technical problems described above, an electro-optical device in accordance with the present invention is provided with the driving circuit described above in accordance with the present invention, so that it permits direct input of a digital image signal, enabling an electro-optical device to be achieved that is capable of providing high-quality gray scale display at relatively low power consumption by using a relatively simple configuration.
To solve the technical problems described above, electronic equipment in accordance with the present invention is provided with the electro-optical device in accordance with the present invention described above, so that it makes it possible to accomplish various types of electronic equipment that has a relatively simple composition, consumes relatively low power, and is capable of providing high-quality gray scale display.